Hydroxypolyamides serve as soluble oligomeric and/or polymeric polybenzoxazole precursors in thermoresistant photoresists for producing heat resistant relief structures; i.e., for the economical direct structuring of organic insulating layers. In addition to a high thermal load capacity (&gt;450.degree. C.) and very good electrial characteristic values, a very high purity is also required of insulating materials which remain in microelectronic components. Thus, the concentration of e.g. chloride ions should lie distinctly below 3 ppm. This is because chloride ions impair the electrical properties of the components and they can negatively influence the long-term stability of the components through corrosion processes.
Soluble, and therefore having good workability, polybenzoxazole precursors in the form of hydroxypolyamides which are converted into high-temperature-resistant polybenzoxazoles by means of a temperature treatment, are known from e.g. EP-PS 0 023 662, EP-OS 0 291 779 and DE-OS 37 16 629. These polybenzoxazole precursors are prepared starting from aromatic diaminodihydroxy compounds and aromatic dicarboxylic acid chlorides. In order to reach the low degree of ionic impurities required in microelectronic engineering, it is usually necessary to perform extensive purifying operations by means of ion exchangers. Such purifying operations involve labor- and cost-intensive processes. Another drawback of such purifying operations is that organic impurities, e.g. amines stemming from anion exchangers, can get into the resinous matter and negatively influence the properties of the resinous matter.
A chloride-free synthesis of polybenzoxazole precursors is known from EP-OS 0 158 726. In this connection, diaminodihydroxy compounds are caused to react with dicarboxylic acids in the presence of a carbodiimide. During this conversion, though, carbamide residues, which remain due to a rearrangement reaction on the resin, cause difficulties. Namely, these residues impair the thermal resistance of the polybenzoxazole precursors and the constitution of layers made out of them.
Accordingly, it is an object of the invention to specify a simple and economical method for preparing hydroxypolyamides in a chloride-free manner.